Image enhancement processing for improving an image by sharpening the image has been hitherto known widely. For example, a conventional television set performs contour compensation for making rising and falling of a video signal, corresponding to a contour of a displayed image, steep. Such contour compensation improves visual image quality by extracting a high frequency component of an input image signal (a luminance signal), amplifying the high frequency component, and then adding the amplified high frequency component to the input image signal. FIG. 12 illustrates diagrams of changes in a waveform indicative of a signal level of an image subjected to the conventional image enhancement processing. FIG. 12(A) is a diagram illustrating a waveform of the signal level of the input image signal in a horizontal direction, particularly illustrating a waveform of a portion corresponding to an edge where the signal level changes in the horizontal direction. FIG. 12(B) illustrates a high frequency component extracted from the input image signal. By amplifying the high frequency component and adding the amplified high frequency component to the input image signal, an output image signal having steep rising of the edge as illustrated in FIG. 12(C) may be obtained.
In recent years, also, a technology called super-resolution for up-converting an input image into a high resolution output image and performing image enhancement processing to the up-converted image has been suggested (for example, see Non-Patent Document 1).